https://github.com/hypothesis/h/commit/46e4bbadc991d550ca9706b5237e35d68675a6cc
made a breaking change to the public API of the endpoint for removing
the current user from a group by renaming the "user" path parameter to
"userid".

Given that this change has been out for some time, I have opted to fix
the client rather than revert the change in the service.

Fixes #774